1. Field of the Invention
One embodiment of the present invention relates to a light-emitting element, a light-emitting device, an electronic device, and a lighting device. In addition, one embodiment of the present invention relates to a lighting system. Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a power storage device, a storage device, a method for driving any of them, and a method for manufacturing any of them.
2. Description of the Related Art
In recent years, research and development of light-emitting elements using electroluminescence (EL) have been actively conducted. In a basic structure of such a light-emitting element, a layer containing a light-emitting substance (an EL layer) is interposed between a pair of electrodes. By applying a voltage between the pair of electrodes of this element, light emission from the light-emitting substance can be obtained.
Since the above light-emitting element is a self-luminous type, a display device using this light-emitting element has advantages such as high visibility, no necessity of a backlight, and low power consumption. Further, such a light-emitting element also has advantages in that the element can be formed to be thin and lightweight, and that response time is high.
It is said that the light emission mechanism of a light-emitting element is as follows: when a voltage is applied between a pair of electrodes with an EL layer including a light-emitting substance provided therebetween, electrons injected from a cathode and holes injected from an anode recombine in the light emission center of the EL layer to form molecular excitons, and energy is released and light is emitted when the molecular excitons relax to the ground state.
The excited states of an organic compound in which molecular excitons are formed include a singlet excited state (S*) and a triplet excited state (T*), and light emission from the singlet excited state is referred to as fluorescence, and light emission from the triplet excited state is referred to as phosphorescence. The statistical generation ratio thereof in the light-emitting element is considered to be S*:T*=1:3. In other words, a light-emitting element containing a compound emitting phosphorescence has higher emission efficiency than a light-emitting element containing a compound emitting fluorescence. Therefore, light-emitting elements containing phosphorescent compounds capable of converting a triplet excited state into light emission has been actively developed in recent years.
Among light-emitting elements containing phosphorescent compounds, in particular, a light-emitting element that emits blue light has not yet been put into practical use because it is difficult to develop a stable compound having a high triplet excited energy level. For this reason, the development of a light-emitting element containing a more stable fluorescent compound has been conducted and high efficiency of a light-emitting element containing a fluorescent compound (fluorescent light-emitting element) has been required.
In the light-emitting element containing a fluorescent compound, triplet-triplet annihilation (TTA) is known as a light emission mechanism capable of converting part of a triplet excited state into light emission. The term TTA refers to a process in which, when two triplet excitons approach each other, excited energy and spin angular momentum are exchanged and transferred to form singlet excitons.
As a compound in which TTA occurs, a compound including an anthracene skeleton is known. Non-Patent Document 1 discloses that the use of a compound including an anthracene skeleton as a host material achieves high external quantum efficiency in a light-emitting element that emits blue light. It also discloses that the proportion of the delayed fluorescence due to TTA to the total light emitted from the light-emitting element using a compound including an anthracene skeleton is approximately 10%.